The present invention relates to a solid state pickup system for picking up an image by use of a color solid state pickup device, and particularly relates to a solid state pickup system giving an effect of a negative spectral sensitivity so as to produce a picture signal having improved color reproducibility.
Concerning conventional solid state pickup systems, a camera having a built-in video tape recorder (VTR) in which CCD image sensors are used for a solid state pickup device, an electronic still camera, a copy machine, and other picture equipments are known.
In such a system, for example, of a single plate type, an image is picked up and converted photoelectrically by a solid state pickup device having a photodetection surface on which an optical filter constituted by very small filters having spectral characteristics of red (R), blue (B) and green (G) or their complementary colors are arranged in a mosaic form, so that electric signals (hereinafter referred to as `color signals`) corresponding to stimulus values of the above-mentioned respective colors are generated. In a system of a multi-plate type, on the other hand, an image is picked up and converted photoelectrically by specific solid state pickup devices having spectral characteristics of red (R), blue (B) and green (G) or their complementary colors. That is, solid state pickup devices, the number of which corresponds to the number of respective colors, produce color signals.
Then, for example, dot phosphors are made to emit light by electronic beams corresponding to the respective stimulus values of color signals R, G and B, so that color reproduction of a subject to be picked up is achieved by color mixture.
As has been well known, reproduction of intermediate colors by color mixture is realized on the basis of the threeway ratio of light intensity among three primary colors (R), (G) and (B) as plotted in the chromaticity diagram shown in FIG. 8, or their complementary colors cyan (C), magenta (M) and yellow (Y). The principle is such that, in the case of R, G and B, it is possible to reproduce a desired intermediate color inside the area (hereinafter referred to as `ideal reproducible area by three primary colors`) of the triangle (the solid line triangle in FIG. 8) taking these primary colors as its apexes, and on the other hand, in the case of C, M and Y, it is possible to reproduce a desired intermediate color inside the area (hereinafter referred to as `ideal reproducible area by complementary colors`) of the triangle illustrated by the one-dotted chain line in FIG. 8. A chromaticity diagram without the complementary colors shown is illustrated in FIG. 10 for the sake of simplicity.
However, when color reproduction is performed by use of color signals obtained by a conventional solid state pickup device photoelectrically converting light from a subject, a real reproducible area is narrower than the above-mentioned ideal reproducible area. For example, the real reproducible area corresponds to the inner area of the triangle having apexes r, g and b illustrated by the dotted triangle in FIGS. 8 and 10.
Here, according to the additive mixture of colors, the following interrelation is established between the three primary colors and their respective complementary colors: ##EQU1## Now further describing the real reproducible area on the basis of the three primary colors (R), (G) and (B), when the above-mentioned dot phosphors of (R), (G) and (B) of a color picture tube are made to emit light by use of electric signals of red (r), green (g) and blue (b) obtained by photoelectric conversion of a conventional solid state pickup device so that color reproduction is performed, or when luminous elements of the three primary colors are made to emit light by use of the above- mentioned electric signals so that color reproduction is performed, a real reproducible area becomes narrower than the ideal reproducible area in FIGS. 8 and 10.
That is, in the prior art, it is impossible to obtain color signals R, G and B corresponding to the three primary colors (R), (G) and (B) positioned at the apexes of the triangle of the ideal reproducible area, and it is possible only to obtain colors (r), (g) and (b) shifted inside the triangle in comparison with their theoretical values, as three primary colors.
It is, therefore, impossible in the prior art to reproduce an intermediate color A in the area inside the ideal reproducible area and outside the real reproducible area in FIGS. 8 and 10, so that an intermediate color A' reproduced by the colors (r), (g) and (b) is regarded as the above-mentioned intermediate color A. As a result, for the intermediate color A of a subject to be recorded, what is in fact reproduced, is, the diluted intermediate color A, nearer white W than the color
The reason why ideal color reproduction cannot be realized will be further discussed with reference to FIG. 9. FIG. 9 shows a color matching function showing an ideal spectral characteristic of a color filter, in which the abscissa and ordinate indicate wave length and stimulus value (lumen) respectively, and a solid line F.sub.R, a broken line F.sub.G and a dotted line F.sub.B indicate the ideal spectral characteristics of red, green and blue filters respectively.
If color mixture is performed with color signals of those ideal characteristics F.sub.R, F.sub.G and F.sub.B, it is possible to reproduce intermediate colors in the ideal reproducible area in FIGS. 8 and 10.
However, in FIG. 9, it is impossible with a conventional optical filter to obtain negative stimulus values .alpha., .beta., .gamma.and .delta. in which F.sub.R &lt;0, F.sub.G &lt;0 and F.sub.B &lt;0, and, therefore, these negative stimulus values (shown in the shaded portion in FIG. 9) are obtained as a zero stimulus value, so that in fact, obtained are characteristics F'.sub.R, F'.sub.G and F'.sub.B in which positive stimulus values are added to the ideal characteristics F.sub.R, F.sub.G and F.sub.B. This is why the narrowed reproducible area indicated by the dotted triangle taking (r), (g) and (b) as its apexes in FIGS. 8 and 10 exists in conventional devices.
Human eyes have a superior color resolution to intermediate colors positioned on the line connecting the blue (B) and the green (G) points in FIGS. 8 and 10 and therefore, it is extremely important to improve color reproducibility in this area.